1. Field of the Invention
The present invention relates to a drive method for a production machine, such as an injection molding machine, which method is preferably used in the case where an actuator provided in the production machine is driven by means of a power drive source.
2. Description of the Related Art
A hydraulic drive apparatus for an injection molding machine disclosed in Japanese Patent No. 3455479 is an example of a conventional drive apparatus which drives a hydraulic actuator provided in an injection molding machine by means of a hydraulic drive source.
The hydraulic drive apparatus disclosed in the patent includes a fixed-displacement hydraulic pump, and a servomotor for driving the hydraulic pump, in which the discharge flow rate and discharge pressure of the hydraulic pump are controlled through control of the rotational speed of the servomotor. Further, a pilot port of a control valve connected to a hydraulic actuator of the molding machine main body is connected to a supply line for working fluid discharged from the hydraulic drive source, via a check valve for preventing reverse flow from the pilot port, and a changeover valve connected in series with the check valve.
Incidentally, the injection molding machine disclosed in the patent includes an injection apparatus and a mold clamping apparatus. In the mold clamping apparatus, a mold clamping cylinder (hydraulic actuator) is driven and controlled by means of the hydraulic drive apparatus so as to clamp a mold. Specifically, in a mold clamping step, high-speed mold closing is performed for a mold in an open state, and then low-speed mold closing is performed when the movable mold half of the mold (movable platen) reaches a low-speed position. Subsequently, high-pressure mold clamping is performed upon closure of the mold. In this case, the drive output for the high-speed mold closing can be set to a ratio (percent ratio) of the rated output (maximum output; 100%) of the hydraulic drive source (hydraulic pump). For example, in ordinary case, a percent ratio of about 90% is typically set. However, when a user wishes to decrease the mold closing speed further, the user can lower the drive output by setting a smaller percent ratio such as 80% or 70%.
However, even when the user lowers the drive output by setting a smaller percent ratio such as 80% or 70%, the actual mold closing speed (mold closing time) achieved by the driven mold clamping cylinder does not drop in proportion to the drive output of the hydraulic drive source, because of influence of, for example, conduit loss (thermal loss) produced within a hydraulic circuit extending from the hydraulic drive source to the mold clamping cylinder. Depending on the configuration and characteristics of the hydraulic circuit, in general, the drop ratio (change ratio) of the mold closing speed (mold closing time) is smaller than that of the drive output of the hydraulic drive source. In other words, even when the drive output of the hydraulic drive source is lowered to a large extent, the mold closing speed (mold closing time) does not drop as much.
FIG. 4 shows the results of an experiment in which in a mold clamping step, motor current I supplied to a servomotor for driving a hydraulic pump of a hydraulic drive source was actually measured with the progress of a molding cycle (molding process). In FIG. 4, Io represents motor current at the time when the drive output was set to 90% output, and Is represents motor current at the time when the drive output was set to 70% output. Further, To represents the mold closing time of a high-speed mold closing section when the drive output was 90% output, and Ts represents the mold closing time of the high-speed mold closing section when the drive output was 70% output. As is apparent from FIG. 4, when the drive output is lowered from 90% output to 70% output, although the motor current I drops greatly, the mold closing time T does not increase as much.
FIG. 5 shows the relation between the motor current I and the closing time T in a comparable manner. When the drive output is lowered from 90% output to 70% output, although the motor current I drops from 4.45 A to 2.65 A, the closing time T increases only slightly, from 0.16 sec to 0.18 sec. That is, although the drop ratio of the motor current I is about 40%, the increase ratio of the mold closing time is about 10%.
As described above, in the case of a hydraulic drive apparatus, in many cases, the actual value of a certain control parameter does not change in proportion to a value set for the control parameter, and may greatly change depending on the configuration and characteristics of the hydraulic circuit and other factors. In addition, a user cannot know this state as clear information. If a user can know the above-described state of high-speed mold closing from accurate information representing the state, the user can compare the state at the time of 90% output and the state at the time of 70% output. Through this comparison, the user may determine that the effect of reducing electricity consumption attained by 70% output outweighs the effect of shortening the closing time attained by 90% output, and accordingly select 70% output. This selection is desirable from the viewpoint of energy saving (the effect of reducing the emission of carbon dioxide). As described above, the conventional hydraulic drive apparatus has not been driven in a proper manner from the viewpoint of energy savings, and there has been room for further improvement.